1. Field of the Invention
The present invention relates to a disk drive having a voice coil motor for driving an actuator, which forces a magnetic head to move rotatably on the surface of a recording medium such as a disk, in order to perform read/write operations. More specifically, the present invention relates to an improvement of a coil structure in the voice coil motor for the disk drive, which allows the actuator to move efficiently.
2. Description of the Related Art
There is recently a tendency to demand, in a computer system, a transfer of large amounts of data at high speed, and therefore, an auxiliary storage device such as a magnetic disk drive is also required to transfer large amounts of data at high speed to exchange data with a host device. To meet this requirement, it is essential for magnetic disk drive, etc., to have a recording medium, e.g., a disk, having a high density recording surface (e.g., a track pitch thereof is less than 10 .mu.m).
A storage device such as a magnetic disk drive executes a so-called head seek operation for moving a head to a designated target track (target cylinder) position from a current track (cylinder) position on a disk by controlling an actuator. When the head is positioned to the designated target track position, a data writing or reading operation is carried out through the head. Such a head positioning control is performed using a digital servo system. In this case, it is also required that the actuator should respond to the digital servo system at high speed and should be controlled efficiently with lower power consumption.
A driving source, e.g. a voice coil motor for driving a conventional actuator for a disk drive will be described with reference to the accompanying drawings.
FIG. 1 is a perspective view showing a construction of a main part of a voice coil motor using a flat coil. FIG. 2 is a schematic front view of FIG. 1 for explaining the operation of the construction shown in FIG. 1. FIG. 3 shows a construction of a main part of a voice coil motor using a square coil. FIG. 4 is a schematic side view of FIG. 3 for explaining the operation of the construction shown in FIG. 3.
First, a voice coil motor using a flat coil will be described with reference to FIGS. 1 and 2. In these figures, a first magnet 3 and a second magnet 4 are placed on a surface of a lower yoke 1 facing an upper yoke 2. Similarly, a third magnet 5 and a fourth magnet 6 are placed on a surface of the upper yoke 2 facing the lower yoke 1 (the third and fourth magnets are not shown in FIG. 1).
The lower yoke 1, upper yoke 2, first magnet 3, second magnet 4, third magnet 5, and fourth magnet 6 constitute a magnetic assembly. A magnetic air gap 7 is created between the first magnet 3 and second magnet 4, and between the third magnet 5 and fourth magnet 6.
Then, a flat coil 8, which is mounted on an actuator that is not shown, is arranged in the magnetic air gap 7.
Next, the operation of the foregoing construction will be described. When an electric current flows through the coil 8, a thrust, i.e., a moving force occurs at the coil 8 arranged in the magnetic air gap 7. Thus, the actuator moves in either of two arrow directions in FIG. 1, by means of such a thrust.
Further, a voice coil motor using a square coil will be described with reference to FIGS. 3 and 4. A yoke 11 comprises a lower yoke 12 that is mounted on a base portion of a disk drive, a vertical yoke 13 extending upward from the lower yoke 12, a middle yoke 14 extending from the substantial center of the vertical yoke 13 in parallel with the lower yoke 12 and in the same direction as the lower yoke 15, and an upper yoke 12 extending from the upper end of the vertical yoke 13 in parallel with the lower yoke 12 and in the same direction as the lower yoke 12. A first magnet 16 is placed on the surface of the lower yoke 12 facing the middle yoke 14. A second magnet 17 is placed on the surface of the upper yoke 15 facing the middle yoke 14.
The lower yoke 12, vertical yoke 13, middle yoke 14, and first magnet 16 constitute a first magnetic assembly. A first magnetic air gap 18 is created between the first magnet 16 and the middle yoke 14. Similarly, the middle yoke 14, vertical yoke 13, upper yoke 15, and second magnet 17 make up a second magnetic assembly. A second magnetic air gap 19 is created between the second magnet 17 and middle yoke 14.
20 denotes a square coil, which is composed of a first portion 21 arranged in the first magnetic air gap 18, a second portion 22 arranged in the second magnetic air gap, and third and fourth portions 23 and 24 that communicate with the first portion 21 and the second portion 22.
Next, the operation of the foregoing construction will be described. When a control circuit of a servo system that is not shown supplies current to a coil 20, a thrust occurs at the first portion 21 arranged in the first magnetic air gap 18 and at the second portion 22 arranged in the second magnetic air gap 19. An actuator moves linearly in either of two arrow directions in FIG. 3.
As described above, a disk drive having a larger storage capacity and making an access at high speed has been required in recent years. Such a larger storage capacity forces the number of disks or heads to be increased, as well as a weight of an actuator. Further, due to the increase of a weight of the actuator, a voice coil motor is required to provide a larger driving force. Furthermore, in order to increase an access speed, a voice coil motor having a larger driving force is necessitated. As strategies for increasing the driving force of a voice coil motor, the following methods are considered;
(1) Increasing a magnetic field strength of a magnetic assembly PA1 (2) Increasing the length of a coil in a magnetic air gap PA1 (3) Increasing an electric current flowing through a coil.
The above-mentioned method (1) requires larger dimensions of magnets, thereby increasing the whole size of a disk drive. The method (2) requires larger dimensions of coil. The method (3) leads to an increase in power consumption.
On the other hand, when a flat coil 8 described in FIGS. 3 and 4 is utilized, as shown in FIG. 5, a thrust occurs only at about 50% of an overall length of the coil arranged in a magnetic air gap 7 (hatched portions in FIG. 5). Further, when a square coil described in FIGS. 1 and 2 is utilized, as shown in FIG. 6, a first portion 21 and a second portion 22 arranged in a first magnetic air gap and a second magnetic air gap 19 occupies only about 50% of the overall length of the coil. In either case of the above-mentioned prior arts, the coil is not utilized effectively.